This invention relates in general to endless metal belts, and in particular to a multilayer endless metal belt assembly which has hardened belt surfaces to prevent galling, and to improve lubrication and wear resistance.
Endless metal belts are known for many purposes, including their use in a nested assembly as drive members for continuously variable transmissions. When used in this manner, an endless metal belt assembly must have certain properties and characteristics to operate efficiently.
An endless metal belt assembly must be constructed of a material which is strong, exhibiting both a high fatigue strength which reduces the likelihood of failure from fatigue fracturing, and high compressive strength and tensile strength, which enables the belt to withstand the demands imposed by the bending stresses inherent in the operation of the dual pulley system of a continuously-variable transmission. The belt material must stretch without yielding and be flexible. It must be durable and exhibit high wear resistance, because replacement is costly and requires that the transmission be non-functional. The belt material must have high processability, and be capable of being fashioned into a very thin layer which can be manufactured to a high precision of circumferential length. In the event of multiple metal belts forming the continuously-variable transmission belt assembly, this high precision of circumferential length for each successive belt is especially critical to the formation of equal gaps between each successive belt of the assembly. There must be minimal friction between belts; thus, the surfaces of the belts must be designed to maintain a lubricated state within the spaces between the belts. Each belt of a multilayered belt assembly must be capable of equal load sharing.
U.S. Pat. No. 3,604,283 to Van Doorne discloses a flexible endless member consisting of one or more layers of steel belts for use with a continuously-variable transmission containing a driving mechanism which comprises a driving pulley with a V-shaped circumferential groove and a driven pulley with a V-shaped circumferential groove. The flexible endless member, which has chamfered (beveled) flanks, interconnects and spans the pulleys. The diameters of the pulleys automatically and steplessly can be varied with regard to each other in such a way that different transmission ratios can be obtained.
U.S. Pat. No. 4,661,089 to Cuypers discloses an endless metal belt for use with a continuously-variable transmission which is strengthened by the incorporation of permanent compressive stresses in the belt's edge zones to reduce the stresses in the edge zones, in particular the tensile stresses caused by the bending stress. The strain on the belt is thereby reduced, and the likelihood of belt breakage caused by hairline cracks occurring from the edges is decreased.
Endless metal belts used for belt drives can be formed by several methods. One manufacturing method disclosed in Metals Handbook, 9th ed., employs a "ring rolling method" wherein a metal, cylindrical tube is cut to a specified length and then an innermost belt is formed on the ring-rolling machine, making the ring wall thinner and the circumferential length longer. Subsequently, a number of additional belts, each with a diameter slightly larger than the diameter of the previously formed belt, may be similarly formed. The belts are then submitted to solution annealing in a vacuum furnace on a stainless steel cylinder, wherein the layered belts are rotated around two pulleys with tension applied in order to adjust the gap between the belts. After the dimensional correction, the layered belt is processed by precipitation-hardening (e.g., 490.degree. C. for 3 hours) and surface-nitriding. Finally, in order to improve lubrication ability between belts, surface profiling is performed, by techniques such as peening, knurling, or preferentially heating or annealing a portion of the surface.
U.S. Pat. No. 4,787,961 to Rush discloses another method of preparing a multilayered endless metal belt. A tensile band set is formed from a plurality of separate looped endless bands in a nested and superimposed relation. The patent states that the bands are free to move relative to each other, even though the spacing between the adjacent bands is relatively small. Such bands may be formed by electroforming at least one band of the bandset.
U.S. Pat. No. 4,067,782 to Bailey et al. discloses a process for preparing endless metal belts intended for use with xerographic machines. An electroforming process employing a core mandrel suitable for chromium plating is used. The core is initially plated with nickel, and then an optional finishing step provides for subjecting the core to an acid dip bath and then plating the core with chromium.
When an endless metal belt assembly is used with a continuously-variable transmission, it experiences frictional stress between belts. In most applications, each belt in a multilayered endless metal belt assembly used with a continuously-variable transmission is moving progressively faster than the next successive inside belt. This type of movement tends to increase the surface area of the adjacent metal belts in contact with each other, producing frictional stress. Therefore, it is desirable to have belts which are flexible, with a lubricous, hard surface.